1. Field of the Invention
The invention relates to a method of operating a semiconductor device as an optical filter and to a semiconductor device to be operated according to this method and wherein the semiconductor device includes a substrate and a monolithically integrated waveguide layer having a branched, simply contiguous structure.
2. Background Information
The publication Electronics Letters 24 (1990), pages 243-244, discloses a semiconductor device which is there operated as a semiconductor laser. The semiconductor device is monolithically integrated on an n-doped indium phosphide substrate. A likewise n-doped buffer layer of indium phosphide extends above the indium phosphide substrate. A waveguide layer of indium gallium arsenide phosphide (InGaAsP) lies on the planar surface of the buffer layer. The waveguide layer has the shape of the letter "Y". Further layers are provided above the waveguide layer. These layers together with the waveguide layer form a Y-shaped mesa above the buffer layer.
Above the waveguide layer, the surface of the semiconductor laser is covered with a metal layer that is sectioned off by three troughs, thus creating four regions into which the semiconductor laser is subdivided. The waveguide layer extends over all regions. In every region, a current can be applied to the metal layer to flow through the waveguide layer to a metal layer on the underside of the substrate which constitutes the ground contact.
If the current in one of the regions exceeds a certain threshold, the transparency current, the waveguide layer in the respective region becomes "transparent", that is light transmissive. If the current increases further, the laser threshold current of the respective region is exceeded, that is, the waveguide layer becomes laser active in this region and produces coherent light.
The publication J. Appl. Phys. 34 (1963), pages 2997-3003, discloses the determination of the line width and the prerequisites for continuous operation of multi-segment lasers. These are discussed on the basis of the marginal conditions for a Fabry-Perot resonator.